1. Field of the Invention
This invention relates to a valve apparatus for precision gas flow rate control which is capable of making accurate controls of the flow rate of comparatively smaller flow rate fluids such as gas.
2. Related Art
When producing products such as semiconductors, it generally becomes necessary to feed, such as, a very small quantity of processing gas under accurate control and, as the means for such flow rate control, such equipment as a precision gas flow rate control system is being used.
The type of gas flow rate control system usually consists of a sensor unit for detection of the flow rate of very small quantity gas, a valve structure and a control circuit thereof. The sensor unit is equipped with a sensor made of a capillary with an electric heating wire coil wound on it and the very small quantity gas of a minute percentage of the total gas flow passes through the capillary but the majority of gas flow is through a by-passing circuit. On the basis of the detection at this sensor unit, the control circuit controls the aperture of the valve with the valve unit to control the flow rate of the gas. In this case, for control of the aperture of the valve, because of the extremely small overall flow of the very small quantity gas, highly accurate control of the aperture of the valve within a stroke of such as tens of micro-meters becomes necessary. For the application, laminated piezoelectric elements are being used generally which can be used as an actuator being capable of causing larger variations in the thrust within a limited stroke.
To describe the structure of existing gas flow rate control systems referring to FIG. 7, a capillary 6 with a diameter of such as 0.5 mm or thereabout to flow a minute percentage of the overall gas flow rate connects both ends of a by-pass 4 of a fluid passage 2 and, around the capillary 6, a pair of electric heating wires 10 of the sensor unit 8 are wound. The pair of electric heating wires possessing a high resistance thermometer coefficient of such as 5,000 ppm/degree and two resistors are combined to constitute a bridge circuit allowing current flow from a constant current circuit 12. Heat is absorbed from the heating wire coil on the upstream side of the fluid passage by the gas flow, while heat is raised in the heating wire coil on the downstream side of the fluid passage, the heat transfer causing unbalance in the otherwise balanced bridge circuit and the potential difference occurring at this time is used as the flow rate signal.
The signal is amplified by the amplification circuit 14 before being input to the comparison control circuit 16 which compares the signal input and the reference flow rate to expand or contract a laminated piezoelectric element 20 of the valve unit 18 thus moving up and down a valve disc, or a diaphragm 22, to obtain an optimum aperture of the valve. The reason why a diaphragm 22 made of thin metal plate is being used as the valve disc is as follows.
With conventional valve units being provided with moving parts in the gas flow passage such as spring, when corrosive gas is used, the spring, etc. corrode or wear to generate particles which mix into the gas flow. Another reason is that, a high degree of cleanliness is being required for semiconductor production processes for the inherent micro-processing, while use of rubber or plastic materials as the valve disc tends to generate particles, such as, of chips which tend to cause more product rejects. However, by use of a metallic diaphragm, a valve disc of a simpler structure but with a higher cleanliness can be obtained.
Meanwhile, as aforementioned, by use of a laminated piezoelectric element 20 to activate the valve disc, or the diaphragm 22, the gas flow rate can be controlled within a very small stroke range of, such as, 100 .mu.m but a laminated piezoelectric element 20 of this type is for a very small stroke and is very expensive, thus restricting the controllable flow rate range of such a valve unit 18 to a very narrow figure and raising the price. It may be possible to use an electromagnetic valve with which stable operation can be expected under comparatively high temperature environments in substitute to the combination of piezoelectric element 20 and diaphragm 22, but with electromagnetic valves of a conventional structure, the bobbin and plunger are exposed direct to gas flow which, when corrosive, generates particles to deteriorate the required high cleanliness.
Whereas, when using a metallic diaphragm as the valve disc, because of its inherently small displacement range, electromagnetic valves cannot provide stable control by its electromagnetic force only. In other words, while a large thrust can be obtained within a very small stroke range with the piezoelectric element 20, such a performance cannot be expected from an electromagnetic actuator and, moreover, since electromagnetic force which is being used by an electromagnetic actuator varies in inverse proportion to the square of the gap distance, the type of an actuator has not thus been able to perform appropriate controls of the aperture of a valve using diaphragm.